Tycho, at the center of the Moon's brightest and craterlets or rilles worth mentioning on Tycho's most extensive system of rays, is the most obvious floor, just scattered rubble piles. The terrain around crater visible on the surface of the full moon, and the outside of Tycho is a different story, however, therefore as good a place as any to start exploring with innumerable craters of all sizes, many of them the southwest quadrant. Tycho is a huge walled in chains, pocking the landscape. One particularly plain, 90 km (56 miles) in diameter, its walls soaring interesting crater chain southwest of the main crater an impressive 3500 m (12 000 ft) above its floor, is made up of at least seven different craters with capped with peaks 1500 m (5000 ft) high. Wilkins overlapping rims, all in a row. The craters at each and Moore describe the walls as consisting of a end of this formation are by far the largest, giving series of straight segments, and indeed this is the chain the appearance of a dumbbell. The many how they appear, though overall the crater outline is craters surrounding Tycho have two characteristics pretty round. The segments were likely produced by in common - most are rather shallow, though they differential slumping of the debris blown free from are not overrun with lava and do not have broken the crater's center at impact after it piled up to make or disturbed walls, and most lack central peaks. the inner walls of the crater. Landslips occurred at Figure 7.1 is an instructive series of four drawings different times and places around the crater walls, demonstrating the changing face of Tycho as the and not in a perfectly smooth circular pattern. Sun rises higher over it. Take your time to carefully explore the structure As we have mentioned, the ray system of Tycho of the inside walls of Tycho, which are amazingly makes this walled plain famous. Wilkins and Moore complex. You will of course want to do this at low found over one hundred rays emanating from the Sun angles. When the Sun is anywhere close to formation. It is no wonder, then, that at full moon being overhead of Tycho, the crater and its rays the rays from Tycho overwhelm many nearby form­ shine with such a blinding light that almost all detail ations, including the walled plain Maginus, causing is lost. The terracing is very extensive, covering the nineteenth-century German selenographer almost half of the floor, and very irregular - instead Johann Madler (1794-1874) to exclaim "the full of perfect concentric circles, the landslips and ledges moon knows no Maginus!" The most prominent of inside the crater are very much broken up into a Tycho's bright rays streaks northeasterly across collection of wriggles, arcs, and short straight seg­ Mare Serenitatis, where it contrasts with the very ments that are almost nowhere continuous to any dark lavas. You can't miss this ray, which intersects great degree. Look for variations in altitude here, the crater Bessel, or the long rays that stretch all the also the contrast between some ridges which are way to Bullialdus (northwest) and Fracastorius (far rounded, and others which are sharp and uneroded­ east). Another bright ray extends to the southwest looking. toward Scheiner. Tycho is noted for its twin central mountain As with many ray systems, the rays of Tycho do peaks. They are not actually perfect twins, as one is not continue up to the very rim of their parent bigger and, at 1500 m (5000 ft), taller than the other, crater, but they are separated from it by a dark and the smaller peak is capped by a prominent "collar" of basalts. Inside this dark ring is a bright craterlet. There are really no other mountains, ejecta blanket, then the crater's rims themselves. 87 P. T. Wlasuk, Observing the Moon © Springer-Verlag London Limited 2000 88 Observing the Moon :Zt':" ~,.. ~~: .:(;: <~~fi?it.. ~.·."': ~~.~.l{{~~~~':;:,~ ...... " . :,<:.~j .~.<~.,:. ..:;.~. :)::<.:.. :~~:.;.::~:.:. " Figure 7.1. Four drawings of Tycho as the Sun rises . Illustrations by Sally Beaumont. There are at least three different theories to explain crater in the first place. Another interesting charac­ the dark collar of Tycho and other craters like it. teristic of rays systems, including Tycho's, is that the The first theory is that the collar is just a contrast rays cannot be traced back to a single point inside effect, a region that is darker than the large amounts the parent crater: instead, that the rays appear to of bright ejecta immediately surrounding the radiate from several different points inside or nearby crater's rim and the bright highland terrain that lies the parent crater, sometimes tangential to it. Why at a greater radius from the crater's rim. The second this should be is a mystery. theory is that Tycho is situated atop another, older Finally, see how far you can trace each ray - you impact site, the dark collar giving away this older will find that they cross all sorts of landforms with impact basin's location. A third theory is that the equal ease, showing that they consist of material that Tycho impact itself brought the dark material to the was deposited on the surface and are not the result of surface from deep within the lunar crust. At any some internal geologic process that brought them to rate, the dark collar of Tycho illustrates just how the surface. Although their intensity may vary along much we have yet to learn about the Moon and its their individual lengths, the rays are usually fairly geologic history. continuous along those lengths; notice also that Like all rays, the rays of Tycho are mere surface some rays are much longer than others. The ray markings, for they cast no shadows even at very low system of Tycho, as well as the fact that its features Sun angles, and so have no appreciable height what­ are very sharp and well defined, show that it is a soever - they are just scourings of the surface by comparatively young, freshly formed crater. This debris ejected from the central crater at the impact hypothesis may have been confirmed by samples of whatever body - asteroid or comet - made that brought back by Apollo 17. Although the Apollo 17 Lunar Features - Southwest Quadrant 89 samples were from many hundreds of kilometers The highlight here is actually Hesiodus A, the away, there is evidence that some samples were of most striking example on the Moon's surface of a ejecta from Tycho, and, if true, the crater would be concentric ring crater. Touching the southwest wall just over 100 million years old - "young" by lunar of Hesiodus, it is a perfectly-formed bowl with a standards. Tycho stands in contrast, then, to the slightly offset secondary rim some way inside the region of the Moon in which it is located - the so­ outer walls, with a nice little group of central peaks called southern highlands - which is undoubtedly at the very center of the concentric walls. Not to be one of the most ancient regions of the lunar surface. confused with the subtle rille on the floor of the Pitatus, a large (80 km, 50 miles wide) lava­ main crater is the Hesiodus Rille, a much longer flooded, lagoon-like ring, is located on the southern and wider rille that begins outside the north rim and shores of Mare Nubium. Its north wall is breached, runs southwest, far across Mare Nubium, onto and the walls can be difficult to trace in their entirety, rough highlands and back down across Palus being much eroded by the lava that spilled over their Epidemiarum. rims. The overall geometry of Pitatus's walls is very Mare Nubium, the Sea of Clouds, is the largest curious, with many undulations, including some mare area in the southwest quadrant, and also one wide, shallow, wriggling valleys on the east and of the "shallowest" of the lunar seas. How do we southeast ramparts. Several nice craters lie atop the know this? Because, unlike many maria, which have walls of Pit at us, one on the west wall having a prom­ only a few craters at their elevated shores where the inent central peak. A broad valley leads through the lava flows couldn't cover them, you can see many west wall to the adjoining walled plain Hesiodus. craters even in the middle of Mare Nubium, though The floor of Pitatus is very smooth and dark, most of these are filled practically to their brims completely flooded by basaltic lava. There are excit­ with lava, and many of their rims are barely visible, ing details to observe on the floor, however - it is far and hence are called ghost craters. It is thought that from featureless. Several small but interesting Mare Nubium is a very ancient impact basin, and it groupings of low mountains lie here and there contains a good number of interesting objects, some inside the walls, especially to the south and west. of which we shall discuss below. There is a central mountain mass just north of dead Longomontanus is an extensive walled plain, center - it looks rather low, but you are just seeing 145 km (90 miles) across, whose largely ruined walls the top of the mountain, the majority of it no doubt are very heavily cratered. Wilkins and Moore buried beneath the lava flow. Be sure to look for describe the walls as "abounding in ridges and rocky three white patches on the floor - two in the south spurs" and as containing "dykes of rock which stand (one southeast of the central peak, another due out very prominently at times," but I am convinced south of it) and one in the northwest corner.